offer some color presets

[ja-he]

inspo: https://matplotlib.org/stable/tutorials/colors/colormaps.html

example for magma:

require("fugitive-blame-heatmap").setup({
  colors = {
    [1] = { value = 0.00, color = "#06051a" },
    [2] = { value = 0.25, color = "#792281" },
    [3] = { value = 0.50, color = "#bb3876" },
    [4] = { value = 0.75, color = "#f56b5b" },
    [5] = { value = 1.00, color = "#fbfbbf" },
  },
})

just went through with a colorpicker, basically since rn there's just an RGB lerp, nicer color-interp can be emulated by specifying more fixpoints